The high data rate demands of modern information processing infrastructure have led to the use of optical data transmission technology for the interconnection of infrastructure components. As the threat to information processing infrastructures continues to increase, the requirements for security become more demanding. Computer systems, communications systems and traffic control systems, all of which are computer processor based, are examples of highly autonomous information processing infrastructures that require the highest level of protection from unauthorized access. In each of these systems, the various components of the infrastructure are accessed by one or more data communications networks. Typically, access control for these systems is provided by internally implemented security software operating integrally within the network. The weakness of such an approach is that the security system is vulnerable to attack from anywhere within the data communications network. There is a requirement for a security system that is not accessible from within the data communications network.